Speedstep is controlled in the BIOS and when the processor detects it is idling it reduces its multiplier and voltage. Completley OS independant.

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Depend, OS can also manage it, I remember XP on my laptop when I was on performance mode then the multiplier was always on max... even on battery! I was able to choose between performance/adaptive/battery-life..

It can be, usually from preinstalled OEM software which make ACPI calls to manage low level functions unique to the laptop, like my netbook has an underclock mode in the taskbar, gotta click max battery life and it dims the display and halfs the clock speed to 750mhz (SLOW!)